Aspirin (acetylsalicylic acid, ASA) has been available for use as an analgesic-antipyretic for almost a century and novel therapeutic applications for this drug, for example in lowering the risk of myocardial infarction or as a prophylaxis against colorectal cancer, continue to be uncovered (Weissmann, G. (1991) Sci. Am. 264, 84–90; Ridker, P. M., Cushman, M., Stampfer, M. J., Tracy, R. P. & Hennekens, C. H. (1997) N. Engl. J. Med. 336, 973–979; Marcus, A. J. (1995) N. Engl. J. Med. 333, 656–658). The acetylation of cyclooxygenases I and II (COX I and II) and the subsequent irreversible inhibition of prostaglandin (PG) and thromboxane biosyntheses are well understood mechanisms of some of ASA's pharmacological actions (Marcus, A. J. (1995) N. Engl. J. Med. 333, 656–658; Herschman, H. R. (1998) Trends Cardiovasc. Med. 8, 145–150). More recently, ASA was found to cause a switch in eicosanoid biosynthesis as the acetylation of COX II changes the enzyme's activity to produce 15R-hydroxyeicosatetraenoic acid from agonist-released arachidonic acid Herschman, H. R. (1998) Trends Cardiovasc. Med. 8, 145–150). Human neutrophils, and other cells possessing 5-lipoxygenase, utilize this substrate via transcellular biosynthetic routes to produce 15-epi-lipoxin A4 (15-epi-LXA4) and 15-epi-lipoxin B4 (15-epi-LXB4) (Serhan, C. N. (1997) Prostaglandins 53, 107–137; Chiang, N., Takano, T., Clish, C. B., Petasis, N. A., Tai, H.-H. & Serhan, C. N. (1998) J. Pharmacol. Exp. Ther. 287, 779–790). These aspirin-triggered lipoxins (ATL) are the endogenous 15R enantiomeric counterparts of lipoxin A4 (LXA4) and lipoxin B4 (LXB4), respectively, and share their bioactivities (Serhan, C. N. (1997) Prostaglandins 53, 107–137(5)).
Unlike other eicosanoids (e.g., leukotrienes, PGs, etc.), which are generally considered local pro-inflammatory mediators, lipoxins (LX) display potent inhibitory actions in several key events in inflammation, such as polymorphonuclear cell (PMN) chemotaxis, transmigration across endothelial and epithelial cells, and diapedesis from post-capillary venules (Serhan, C. N. (1997) Prostaglandins 53, 107–137(5)). LX are generated in several pathogenic scenarios in vivo, for example: in lung tissue of patients with severe pulmonary disease; and by PMN from patients with asthma or rheumatoid arthritis, where their presence is proposed to be linked to long-term clinical improvement (Lee, T. H., Crea, A. E., Gant, V., Spur, B. W., Marron, B. E., Nicolaou, K. C., Reardon, E., Brezinski, M. & Serhan, C. N. (1990) Am. Rev. Respir. Dis. 141, 1453–1458; Chavis, C., Chanez, P., Vachier, I., Bousquet, J., Michel, F. B. & Godard, P. (1995) Biochem. Biophys. Res. Commun. 207, 273–279; Chavis, C., Vachier, I., Chanez, P., Bousquet, J. & Godard, P. (1996) J. Exp. Med. 183, 1633–1643; Thomas, E., Leroux, J. L., Blotman, F. & Chavis, C. (1995) Inflamm. Res. 44, 121–124). Interestingly, ATL show an even greater level of inhibition than native LX in preventing neutrophil adhesion, where they are ˜twice as potent (Serhan, C. N. (1997) Prostaglandins 53, 107–137). ATL are also more potent inhibitors of microbial induction of cytokine release. Specifically, 15-epi-LXA4 showed greater inhibition than LXA4 of S. typhimurium-induced secretion and gene regulation of the potent leukocyte chemoattractant IL-8, generated by intestinal epithelial cells (Gewirtz, A. T., McCormick, B., Neish, A. S., Petasis, N. A., Gronert, K., Serhan, C. N. & Madara, J. L. (1998) J. Clin. Invest. 101, 1860–1869). It is therefore likely that, in addition to the inhibition of prostaglandin formation, the benefits of ASA therapy also result from the triggering of novel anti-inflammatory lipid mediators that act locally to down regulate leukocytes.